The present invention relates to a stapler for driving staples into an object, such as a bundle of paper sheets, said stapler having a frame, a magazine arranged in the frame and containing substantially U-shaped staples, and a drive element adapted to push a staple out of the magazine and drive its legs through the object.
Prior-art staplers have a fixed anvil which is so positioned that the free leg ends of a staple discharged from the magazine will strike, when the staple is driven into the object, against the anvil which then bends the staple legs driven through the object, against the object in a direction towards each other. The staple thus bent exhibits, in the bent portion of each leg, a curve or bulge making the staple considerably thicker at its ends, as seen in the driving-in direction of the staple, than in its central portion where the staple legs engage the rear side of the object. Such a thickness increase is of course not desirable. For example, it is inconvenient when several bundles of an optional number of paper sheets stapled together are inserted in a binder, since the thickness of the collected bundle becomes considerably greater at the upper left-hand corner where the staples are usually placed when stapling paper sheets.
To overcome this problem, it has been proposed to replace the fixed anvil with a clinching mechanism which bends (clinches) the staples in such a manner that they will have a uniform thickness over their entire length. Known clinching mechanisms rely on one movable anvil member for each staple leg.
A known clinching mechanism will now be described with reference to FIG. 1, illustrating only those parts of a known stapler which are essential to the present description. In FIG. 1, the stapler is shown in a position in which its drive element 1 is driving a U-shaped staple 2 into a bundle 3 of paper sheets placed in the stapler. The clinching mechanism of the stapler comprises two anvil members 4 which are arranged substantially symmetrically with respect to the staple 2 and which each cooperate with one leg 2a of the staple 2. Each anvil member 4 is pivotable about a shaft or pin 5 perpendicular to the plane of the staple 2. As illustrated, the shafts 5 are disposed in the upper portion of the anvil members 4 on each side of the staple 2, i.e. outside its legs 2a. When the staple 2 is driven into the bundle 3 of paper sheets, its free leg ends will encounter, as shown, an oblique surface 6 of the respective anvil member 4, such that the legs 2a are bent slightly inwards, as the staple 2 is driven further into the bundle 3.
When the staple 2 has been driven completely into the bundle 3 of paper sheets, i.e. when its web portion engages the top side of the bundle, the anvil members 4 are pivoted by means of an operating slide 7, which is moved upwards, about the shafts 5 in the direction of the arrows indicated within the anvil members, to bend the legs 2a of the staple 2 into abutment against the underside of the paper bundle 3. In this manner, the staple 2 will be bent without any thickened portions of the type described above occurring at the ends of the bent staple.
Clinching mechanisms of this type are disclosed, e.g. in WO 90/08015, U.S. Pat. No. 4,593,847 and U.S. Pat. No. 4,449,661.
Since, as is often the case, the motion of the operating slide 7 is synchronised with the motion of other components, such as the drive element, included in the stapler, and the slide 7 therefore is mechanically connected to these components, it is difficult to design the clinching mechanism of FIG. 1 in such a manner that the upward displacement of the slide 7 stops in a well-defined turning position, which is necessary for achieving optimum bending results. The difficulties in this respect are caused by tolerance variations of the different components, and by the components "settling" when the stapler has been used for some time.